Aircraft: B757
Phase of flight: Cruise
Anomaly: Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor

Our maintenance history indicated the airplane had four recent write-ups; including a diversion and 'info only' comment relating to an odor in the cockpit. The most recent maintenance action taken was a replacement of the weather radar unit. A subsequent flight was flown and an 'info only' comment was made that 'no fumes' were noticed on their flight. En-route airports had good weather and we felt comfortable that the maintenance action corrected the problem. [About an hour into flight]; we noticed an acrid odor. Knowing the airplane history with the weather radar (which we had on); we confirmed with Dispatch that there was no convective weather near our position and was told none. We turned the weather radar off and the odor dissipated. Five to six minutes later; the odor re-appeared and because of the way the sun was shining in the cockpit; I happen to see what appeared as a very thin layer of haze. We began discussing a diversion and referenced the QRH when I noticed visible smoke coming from the First Officer's instrument switching panel area. I announced 'we have visible smoke'; we then donned our Oxygen mask and established communications. The First Officer was the pilot flying; we declared an emergency and initiated our diversion to the airport. I contacted Dispatch through ACARS and verbally discussed our intentions to divert to the airport. I briefed the flight attendants (using the emergency brief acronym). I followed the Smoke; Fire or Fumes QRH while the First Officer flew the airplane and was coordinating our arrival with Approach. Upon reaching the step to turn the pack selectors OFF; the smoke and odor had completely dissipated. Because we were on a high left downwind position at this time and setting up for the final approach; we abandoned the checklist and set up for the landing. We landed approximately 8;000 LBS. overweight. It was an uneventful landing. ARFF was standing by and reported nothing unusually from their vantage point. We taxied to our gate and deplaned the passengers. I felt we received good support from Dispatch; ATC; ARFF and most certainly the crew. We incurred an immediate action emergency resulting from smoke in the cockpit which required a declaration of an emergency and an immediate descent and landing into the airport. The events that lead to this situation began when we began to notice an acrid smell in the cockpit. A previous write up associated this issue with the weather radar; therefore we secured the radar at this time. The odor cleared and we assessed the situation was resolved. The Captain informed Dispatch of the issue. Maintenance was informed and weather along our route was assessed. In the course of time in which the communication and coordination was accomplished (approximately 5 minutes) we began to notice what appeared to be pieces of wire insulation proceeding from the Captain's eyeball vent followed shortly by the return of the odor; followed by visible smoke proceeding from above the First Officer's instrument selector panel. We initiated the Smoke; Fire or Fumes immediate action procedures. Once communications were established the Captain confirmed that I; the First Officer; had the airplane and ATC and he would conduct the Smoke; Fire or Fumes checklist. He also instructed me to declare an emergency and arrange for an immediate divert and landing into the airport with emergency equipment standing by. The Center gave initial instructions for direct VOR direct the airport with a clearance to descend to 16;000 FT followed by a descent to 14;000 FT and direct another VOR followed by a clearance direct to the airport. During the descent the Captain conducted the checklist and at various intervals inquired of our progress to the airport. He also kept me informed of his status in the checklist. At a certain point the Captain asked me to confirm with him that the smoke and odor had indeed cleared in which I answered in the affirmative. We removed the masks and began coordinating our arrival into the airport. The airport was experiencing relatively strong winds with gusts from the south. Due to the heavy weight and wind correction Vref speed exceeded flaps 30 approach speed so flaps 25 was briefed and selected for landing. Runway 16L was assigned which also was the most appropriate runway for our situation. A normal stabilized approach and landing was made with maximum use of reverse thrust. The brakes were not required nor applied until just prior to 80 KTS. Center handled the emergency very well. Giving us clear and concise directives made navigating and loading the box for arrival more simple in a high workload environment. Clearing us quickly to lower altitudes placed us in a better position to deal with the smoke if it became a greater concern than locating and fighting its source. It seemed to be a very short time from the point the emergency was declared and we arrived at the gate. In retrospect I am critical though; of not obtaining the landing performance data required when diverting and/or landing overweight. This was not intentional; it occurred as a result of operating in a high workload environment and where time is a factor. Adding a note stating 'landing performance data must be evaluated' to any procedure requiring to land at the nearest suitable airport should be considered. Finally; include in the dispatchers' procedures when these types of emergencies occur to offer assistance in obtaining landing performance data. This would be a great back up to the crew as they are dealing with an emergency in the fog of war. B757 flight crew experiences smoke in the cockpit one hour into flight and diverts to the nearest suitable airport. QRH procedures eliminate the smoke prior to landing.